This invention relates generally to tubes used in heat exchangers for transferring heat between a fluid inside the tube and a fluid outside the tube and to a method of manufacturing such tubes. More particularly, the invention relates to a heat transfer tube having an internal surface that is capable of enhancing the heat transfer performance of the tube and offering improved workability when compared to prior art tubes. Such a tube is adapted to use in the heat exchangers of air conditioning, refrigeration (AC&R) or similar systems.
Designers of heat transfer tubes have long recognized that the heat transfer performance of a tube having surface enhancements is superior to a smooth walled tube. A wide variety of surface enhancements have been applied to both internal and external tube surfaces including ribs, fins, coatings and inserts, to name just a few. Common to nearly all enhancement designs is an attempt to increase the heat transfer surface area of the tube. Most designs also attempt to encourage turbulence in the fluid flowing through or over the tube in order to promote fluid mixing and break up the boundary layer at the surface of the tube.
A large percentage of AC&R, as well as engine cooling, heat exchangers are of the plate fin and tube type. In such heat exchangers, the tubes are externally enhanced by use of plate fins affixed to the exterior of the tubes. The heat transfer tubes also frequently have internal heat transfer enhancements in the form of modifications to the interior surface of the tube. One very effective internal surface enhancement in current use is a pattern of ribs extending from the tube inner wall and running parallel or nearly so to the longitudinal axis of the tube. Not only does the tube have good heat transfer performance, it is also relatively easy to manufacture, particularly by a process of roll embossing the enhancement pattern on to one side of a metal strip, then roll forming the strip into a tubular shape and welding the resulting seam.
In a typical plate fin and tube type heat exchanger, there are many tubing joints. These joints are usually made by enlarging the end of a first tube so that the inner diameter of the enlarged section is slightly larger than the original outer diameter of the tube. Then the end of a second tube is inserted into the enlarged section of the first tube and the two tubes are joined by a process such as brazing, welding or soldering.
The usual method of enlarging a tube end is by mechanical means such as inserting a belling or flaring tool into the tube. The enlarging process imposes stresses in the tube wall. These stresses can cause the tube wall to split, particularly if the tube is made of a relatively soft metal such as copper or an alloy of copper as is generally the case with the tubing used in AC&R heat exchangers. A tube having an enlarged end that has serious splits must be scrapped. The splitting problem is especially pronounced in tubing having the longitudinal ribs described above.